Rotation Time Optimization of FDDI Networks

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For the token ring to work in a fault tolerant mode, it must know whether it is physically connected to its neighbor ^[6], and the sender must examine the acknowledgement bit added by the receiver before it removes the packet from the ring ^[7]. If a node is not connected to its neighbor, it re-transmits the token it just received back in the direction from which it came, and the packet travels around the ring in the opposite direction shown in Figure 1. The cable break may occur in such a location that the packet must travel through the master node again before it reaches the receiver node; thus the master must examine the acknowledgment bit before removing the packet from the ring and passing the token to the next node. If a node does not receive a token within some maximally specified time, it generates a token claim algorithm whereby a special frame is generated and passed around the ring. If this special frame makes its way back to the originating node, that node now has the token. This makes the token ring useful even if more than one cable break occurs in the ring because each of the subsections can still pass data between the connected nodes.

1.1. Parameters of Time Token Protocol

(a) TRT (Token Rotation Time): It is the time taken for a token to go around or it is a time of last rotation of token or is the time interval between two successive receptions of the token by a station ^[10].

(1)

(b) TTRT (Target Token Rotation Time): It indicates the expected time of each token rotation ^[9]. The value of TTRT is negotiated during initialization and set to the maximum desired rotation time.

For asynchronous traffic i.e. Computer Data. Applications that do not have time constraints use the asynchronous mode ^[10].

: No Transmission;

:Transmission limited to

For synchronous traffic i.e. Voice Data:

Transmission, but THT is small and fixed so: worst case

Synchronous frames are given higher priority over asynchronous frames ^[9].

(d) Station Delay: It is time needed for a station to read and regenerate.

(e) Ring Latency: It is time taken by bit to go around. It is combination of total propagation delay and sum of station delays.

The performance of an FDDI LAN depends upon configuration and workload parameters such as the extent of the ring, the number of stations on the ring, the number of stations that are waiting to transmit, and the frame size. In addition, one key parameter that network managers can control to improve performance is the target token rotation time (TTRT). Setting the TTRT at 8 milliseconds (ms) provides good performance over a wide range of configurations and workloads ^[5]. Fuzzy FDDI approach proposed by Kirov ^[8] can be implemented into each network node to control the access delay, THT, throughput and thus improving the network performance. Cobb and Lin ^[9] introduced a modified timed-token FDDI protocol. Both approaches can slightly improve the network performance however; they cannot achieve multi Gb/s throughput and full utilization of the bandwidth of the optical fiber.

2. Optimization of TTRT in FDDI Networks

For optimization of the performance of FDDI networks, specified setting the TTRT value should be taken into consideration. First of all, the token rotation time is calculated which can be as long as two times the target. Thus, a synchronous station may not see the token for . Therefore synchronous stations should request a TTRT value of one half the required service intervals. TTRT should allow at least one maximum size frame along with the synchronous time allocation, if any which can be shown by the equation (2).

(2)

The equation-(2) states that minimum value of the TTRT is explained by equation (3)

(3)

The maximum size frame on FDDI is 4500 bytes (0.360 ms), the maximum ring latency is 1.773 ms and the token time is 0.00088 ms. This rule, therefore, prohibits setting the TTRT at less than 2.13ms plus the synchronous allocation. Over allocating the synchronous bandwidth, in contradiction to this rule for requisite frame size, results in unfairness. Also, no station should request a TTRT less than T_min, which is a station parameter. The default maximum value of T_min is 4 ms. Assuming that there is at least one station with T_min=4 ms, the TTRT on a ring should not be less than 4 ms. Another parameter to be considered is that, no station should request a TTRT more than T_max. The default minimum value of T_max is 165 ms. If there is at least one station with T_max=165 ms, the TTRT on a ring cannot be more than this value. In addition to these rules, the TTRT values should be chosen to allow high-performance operation of the ring ^[11].

3. Simulation and Results

Figure 3.1. Efficiency vs Ring Latency for T=165ms and D=80ms

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Figure 3.2. Efficiency vs Ring Latency for T=165ms and D=40ms

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Figure 3.3. Efficiency vs Ring Latency for T=165ms and D=4ms

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Figure 3.4. Efficiency vs Ring latency for T=165ms and for optimized latency

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Keeping the above point into consideration we have calculated some results shown in Figure 3.1-3.12 on software MATLAB 7B of Mathworks. Initially we have fixed the value of maximum value TTRT to165; number of stations is fixed to 100 and the ring latency of the system is varied as 80, 60, 40, 20, 4, 1.773 ms. These results show the optimized results for the efficiency when the TTRT value is 165 ms and the ring latency (D) is fixed to 1.773ms. Thus we can optimize the FDDI network by fixing the value of TTRT value to be equal to 165 ms and reducing the ring delay or ring latency to as low as 1.773ms.

Figure 3.5. Efficiency vs No. of Station for D=80ms and T=165ms

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Figure 3.6. Efficiency vs No. of Station for D=40ms and T=165ms

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Figure 3.7. Efficiency vs No. of Station for D=4ms and T=165ms

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Figure 3.8. Efficiency vs No. of Stations for T=165ms and for optimized latency

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Figure 3.9. Efficiency vs TTRT Value for D=80ms and T=165ms

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Figure 3.10. Efficiency vs TTRT Value for D=40ms and T=165ms

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The results in Figure 3.1-3.12 show the relationship of the efficiency with ring latency, TTRT value and number of stations. We have fixed the value of TTRT to its maximum value which is optimized to 165ms and varied the value of ring latency and reduced its value from 80ms to 4ms and then 1.773 ms which is the optimized value of ring latency. The combination of maximum value of TTRT value (i.e. 165ms) and optimized value of ring latency (1.773ms) has optimized the results to produce the high efficiency of FDDI networks. With these optimized results the value of Efficiency was produced to be 98.9%. In this way we have optimized the performance of FDDI network by optimizing the value of efficiency with TTRT value and latency. This optimization technique results in improved performance of the FDDI Networks. Some of the optimization steps include the adjustment of TTRT value and ring latency. The synchronous stations should request a TTRT value of one half the required service intervals. TTRT should allow at least one maximum size frame along with the synchronous time allocation, if any. The maximum size frame on FDDI is 4500 bytes (0.360 ms). The maximum ring latency is 1.773 ms and token time is 0.00088 ms.

Figure 3.11. Efficiency vs TTRT Value for D=4ms and T=165ms

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Figure 3.12. Efficiency vs TTRT for T=165m sand for optimized latency

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